Apparatus for zone melting of semiconductor bodies through high-frequency heating



Nov. 12, 1968 REGNER ETAL. 3,410,945

APPARATUS FOR ZONE MELTING OF SEMICONDUCTOR BODIES THROUGHHIGH-FREQUENCY HEATING Filed Oct. 8, 1965 2 Sheets-Sheet 1 INVENTORSKa/e/ 369m; Jam fez/a BY .sak

MM may Nov. 12, 1968 K. REGNER ETAL 3,410,945

APPARATUS FOR ZONE MELTLNG OF SEMICONDUCTOR BODIES 2 Sheets-Sheet 2THROUGH HIGH-FREQUENCY HEATING Filed Oct. 8, 1965 3% 0 5 M? W m2.

United States Patent 3,410,945 APPARATUS FOR ZONE MELTING OF SEMICON-DUCTOR BODIES THROUGH HIGH-FREQUENCY HEATING Karel Regner and JosefPetrasek, Prague, Czechoslovakia, assignors to CKD Praha, oborovypodnik, Prague, Czechoslovakia Filed Oct. 8, 1965, Ser. No. 494,025Claims priority, application Czechoslovakia, Oct. 17, 1964, 5,749/64;Apr. 7, 1965, 2,281/65 7 Claims. (Cl. 13-1) ABSTRACT OF THE DISCLOSUREIn a zone melting apparatus there are provided an enclosure and a bell,the latter being on top of the former and being separated from theformer by a base wall which is slidably arranged and adapted to providewhen it engages a rim of the bell a vacuum-tight seal between enclosureand bell. The bell defines a melting chamber. A rod-shaped object whichis to be heated to its melting point is held within the bell. Aninduction coil surrounds said object and is slidable therealong. Aflexible cable passes through a bushing in the enclosure wall andconnects the power supply to a coaxial lead of the coil. A carrierwithin the enclosure is hydraulically operated to be raised and to movealong with it the base wall and the induction coil. The rod-shapedobject is rotated as the induction coil moves along said object. Whenthe base wall sealingly engages the bell, the melting chamber isevacuated. When the induction coil is deenergized, and the vacuum in thesealed melting chamber broken by opening the chamber, the carrierreturns to its starting point and a new cycle may begin. The rods to beheated are introduced into the enclosure through an opening therein.

This invention relates to the zone melting of semiconductor bodies, andmore particularly to an apparatus for the zone melting of rod-shapedsemi-conductor bodies by a high-frequency electric field.

It is known to pass semi-conductor rods through induction coilsenergized by high frequency current so that the portion of the rodcontained within the coil is heated to its melting point by the inducedcurrent. Passage of the molten zone along the rod causes the impuritiesin the material to congregate at one end of the rod where they can besawed otf.

The semi-conductor body must be kept in a vacuum or other protectiveatmosphere while being subjected to zone melting, and the necessity ofperforming the process in a sealed chamber presents numerous problemswhich have not heretofore been solved in a fully satisfactory manner.

If the induction coil is fixedly mounted and the semiconductor rod ismoved through the coil, electrical connections to the coil can beprovided in a very convenient manner, but the sealed chamber must have alength twice that of the longest rod to be treated and additionallyallowing for the holders which secure the two ends of the rod. Chambersas long as two meters have been built for zone melting with a stationaryinduction coil. Such a long chamber is not only costly to build anddifiicult to handle, but is diflicult to clean of the impurities whichtend to accumulate on the chamber walls.

The known devices which employ a movable induction coil traveling alonga rod held stationary in a suitable holder are of two types. In onetype, the rod to be treated is enclosed in a shell in which thenecessary atmosphere is maintained, while the coil envelops both theshell and the rod. The presence of the shell between the coil and theobject to be treated, however, prevents full utilization of the powerinput.

ice

In the other known type of zone melting apparatus employing a movingcoil, the coil is enclosed in the sealed chamber with the rod to betreated, but problems arise in the connection between the coil and itspower supply which is necessarily located outside the chamber. Thesupply leads are quite long, and their impedance changes as they movewith the coil in the chamber.

It is an object of this invention to improve the last-mentioned type ofzone melting apparatus, and to overcome the problems inherent in theconventional power leads.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings in which FIG. 1 is a partialsection elevation with a diagrammatical representation of currentsupply;

FIG. 2 is a plan view with the bell turned aside, diagrammaticallyrepresented by dashed contours;

FIG. 3 is an enlarged detail of the high-frequency circuit upper endtogether with a portion of the rod to be treated.

Referring to the drawing in detail, there is seen an enclosure 10 whosetop is partly open. A heavy column 12 rises from the bottom of theenclosure through the space enclosed thereby, and projects upwardthrough the closed portion of the enclosure top 14. A sleeve 23' isrotatably mounted on the free top end 13 of the column 12 which is ofreduced diameter, thus securing the sleeve against downward movement.

A bracket 22 fixedly attached to the sleeve 23 carries a downwardly openbell 20. It will be understood that the bell is equipped with a jacketconnected to a source of cooling water, as is conventional in this art,but the jacket has been omitted from the drawing for the sake ofclarity. A window 21 sealed to the bell 20 permits observation of theinterior of the bell. A duct 26 flanged to the bell carries a dilfusionpump 25, the arrangement being such that the pump approximately balancesthe weight of the bell 20, and the center of gravity of the elementssupported on the bracket 22 is located close to the vertical axis of thecolumn 12.

A resilient sealing ring 48 on the free rim of the bell 20, this rimabutting against a covering margin 15, provides a vacuum-tightconnection between the bell and a base wall 40 in the illustratedposition of the apparatus. The base wall is mounted on a sleeve 45 whichis freely slidable on the column 12 within the enclosure 10, and is heldin the illustrated position against the evacuated bell 20 by externalair pressure in the enclosure 10 so as to close the open top of theenclosure 10. Two vertical openings through the wall 40 respectivelyreceive sealing bushings 46, 47.

A vertical stand 41 on the base wall 40 within the bell 20 carries abracket 42 on its free top end. An upper rod holder 61 depends from thebracket 42. The stem of a lower rod holder 62 vertically passes throughthe sealing bushing 47, and is attached to an electric motor 44 which issecured to a bracket 43 in the enclosure 10. The two ends of a siliconrod 60 or other semi-conductor object of similar configuration arerespectively held by the holders 61, 62 in a known manner, the holder 61being rotatably mounted on the bracket 42. The device which adjusts thevertical spacing of the holders 61, 62 during zone melting has not beenshown since it may be entirely conventional.

A vertical piston 52, comprising rigid conductors 71, 72 is movablyreceived in the sealing bushing 46. The upper piston end is providedwith a collar 53 to limit the downward movement of the piston. The lowerpiston end is fixedly attached to a carrier bracket 50 integral with asleeve 51 which is freely slidable on the column 12 under the sleeve 45.The rigid conductors 71, 72 are fixedly secured in the piston throughthe medium of a plug 55 in the usual manner (see FIG. 3). An inductioncoil 70 which coaxially envelops the rod 60 is integral with the tworigid conductors 71, 72. The conductor 71 is grounded through the column12, and a bank of capacitors 73 is interposed between the conductor 72and ground at the bottom end of the piston 52 in the enclosure 10.

The high-frequency power supply for the oscillator circuit constitutedby the coil 70, the capacitor 73, and the conductors 71, 72 have beendiagrammatically illustrated, only the power source 75 comprising anoutput tube being shown in the drawing. A flexible cable 74 passingthrough a bushing 16 in the enclosure wall connects the power supply tothe cable 72. The piston rod 34 of a piston 33 adapted to slide within ajack 30, is attached to a lug 54 in the carrier bracket 50. To the upperand lower ends of the jack 30 respective pressure fluid conduits 31 and32 are connected. An air lock 11 on the enclosure permits material to beintroduced into the enclosure 10 where it may then be handled by anoperator placing his hands from the outside into gloves 17 (17) whichare sealed to the inner wall of the enclosure 10, only one glove beingvisible in the drawing.

The afore described apparatus is operated as follows:

The coil 70 is energized by the power supply while a vacuum is beingmaintained by the pump in the sealed chamber enclosed by the bell 20 andthe base wall 40. Hydraulic fluid is admitted through a conventionalcontrol valve (not shown) and the conduit 32 into the jack so as slowlyto raise the carrier bracket 50, whereby the coil 70 passes axiallyupward along the rod 60, sequentially melting portions of the rod in theusual manner. The rod is rotated during melting by the motor 44, andnecessary adjustments are made in the spacing of the holders 61, 62 inorder to maintain the original diameter of the rod during melting bymeans of the nonillustrated device referred to above.

When the coil 70 has completed its traverse of the rod 60, the sleeve 51abuts from below against the sleeve 45, and holds the base Wall when thecoil 70 is deenergized, and the vacuum in the sealed chamber is brokenby opening of a non-illustrated vent cock.

When the hydraulic controls are set to drain fluid from the jack 30, thecarrier and the base 40 with the devices supported thereon descend tothe illustrated position of the carrier 50, the coil 70 still beingclosely adjacent the bracket 42. An operator may now place his hands inthe gloves 17 and replace the treated rod by a fresh rod which isintroduced into the interior of the enclosure 10 through the air lock11.

The hydraulic control valve is next reversed, and the carrier 50'together with the base wall '40 are raised until the base wall sealinglyengages the rim of the bell 20, whereupon the sealed chamber so formedis evacuated, and the base wall 40 is held in place by the vacuum whilethe carrier 50 is returned to the illustrated starting position. A newcycle may begin.

The melting chamber is thus opened and closed by the same hydraulicmotor which also moves the coil 70. The provision of a single motor forboth purposes reduces the bulk, and particularly the overall height ofthe apparatus, and contributes to its stability and freedom fromvibrations, a serious source of difliculties in some known zone meltingdevices.

It will be understood that a non-illustrated pin passes through radiallyaligned openings in the sleeve 23 and the column 12 under all normaloperating conditions of the apparatus to prevent pivotal movement of thebell 20 on the column 12. When it is desired to clean the bell, thecarrier 50 and base wall 40 are fully lowered, the pin is withdrawn, andthe bell is swung on the column 12 until it clears the enclosure 10,whereby free access to the interior of the bell is had from below, andimpurities scraped from its inner wall may be dropped into a suitablyplaced container.

The interior of the enclosure 10 is supplied with dry, filtered air in amanner not further illustrated to avoid contamination of the rods 60. Asupply of several rods to be treated is normally introducedsimultaneously into the enclosure through the air lock 11, and all rodsare withdrawn together after treatment to minimize the hazards ofcontamination during transfer. Obviously, mechanized rod insertion androd removal devices may be installed within the enclosure 10 in a mannernot in itself relevant to the instant invention. t

The apparatus of the invention combines the advantages of theconventional moving-rod zone melting de- Vices with those of the knowndevices using a moving coil. The power supply is connected with theoscillator circuit by a flexible conductor, yet the heating output andthe frequency of the oscillator are stable. The variations in theimpedance of the flexible conductor 74 during coil movement do notsignificantly affect the stability of the operating parameters becausethe current in the conductor 74 is small as compared to the currentflowing in the oscillator circuit. The elements of the latter, namelythe coil 70, the capacitor bank 73, and the conductors 71, 72 rigidlymaintain their spatial relationship through all phases of the operatingcycle.

The elements which move during opening and closing of the sealed chamberin the apparatus of the invention are relatively light. The heavy bellwhich has to withstand most of the atmospheric pressure against theevacuated apparatus, and further carries the heavy water jacket (notshown) remains stationary during normal operation of the apparatus. Itdoes not move vertically under any condition. It is readily pivoted byhand for cleaning and similar maintenance purposes. No special devicesare needed for holding the base wall to the bell.

The provision of a movable base wall permits all manipulations of therods 60 to be performed in the enclosure 10 which may be made gas tight,or which may be flooded with clean dry air or other gas at a suflicientrate to prevent entry of ambient contaminants such as dust through smallopenings.

The modifications of the illustrated apparatus necessary for use of adifferent protective atmosphere will be obvious.

It should be understood, of course, that the foregoing disclosurerelates only to a preferred embodiment of the invention and that it isintended to cover all changes and modifications of the example of theinvention herein chosen for the purpose of the invention which do notconstitute departures from the spirit and scope of the invention setforth in the appended claims.

What is claimed is:

1. In a zone melting apparatus, in combination:

(a) wall means defining a closed chamber, said wall means including 1) abase wall and bell means, said base wall being formed with an opening,

(b) moving means for moving said base wall and said bell means relativeto each other toward and away from a position wherein said base wall andsaid bell means sealingly engage each other to define said chamber,

(c) a fixed support, said base wall and said bell means being mounted onsaid support,

(d) holder means in said chamber for holding a rodshaped object in sucha manner that the longitudinal axis thereof extends in a predetermineddirection,

(e) an enclosure, said moving means including (1) means for moving saidbase wall in said predetermined direction on said support inward of saidenclosure when said base wall is moved away from said bell means, and

(2) air lock means on said enclosure for giving access to said holdermeans when the same is moved into said enclosure with said base wall,

(12') elongated lead means movable in bearer means in said chamber,

(g) actuating means for moving said lead means longitudinally in saidpredetermined direction,

(h) an induction coil fixedly mounted on said lead means in saidchamber, said coil being positioned to envelop a rod-shaped object heldby said holder means,

(i) a capacitor fixedly mounted on said lead means, said capacitor, saidlead means, and said coil being conductively connected to constitute anoscillator circuit, and

(j) means for supplying power to said circuit.

2. In an apparatus as set forth in claim 1, a carrier mounted on saidsupport and operatively connected to said actuating means for movementthereby in said direction, said lead means being attached to saidcarrier.

3. In a zone melting apparatus, in combination:

; (a) a support;

(b) bell means and a base wall mounted on said support, said base wallbeing movable in a predetermined direction on said support toward andaway from a position of sealing engagement with said bell means in whichsaid base wall and said bell means jointly define a closed chamber, saidbase wall being formed with an opening extending therethrough in saiddirection;

(0) pump means for evacuating said chamber;

(d) a carrier mounted on said support outside said chamber;

(e) a lead extending through said opening, said lead being elongated insaid direction and having respective portions in said chamber andoutside said chamber, the latter portion being mounted on said carrier;

(f) actuating means for moving said carrier on said support in saiddirection;

(g) holder means on said base wall for holding an elongated object insuch a manner that the object is elongated in said direction;

(h) an induction coil mounted on the portion of said lead in saidchamber and positioned to envelop an elongated object held by saidholder means;

(i) a capacitor mounted on said lead; said capacitor, said lead, andsaid coil being conductively connected to constitute an oscillatorcircuit; and

(j) power supply means for said circuit.

4. In an apparatus as set forth in claim 3, said lead being rigid.

5. In an apparatus as set forth in claim 4, said holder means includingmeans mounted on said base wall for rotating an elongated object held bysaid holding means about the longitudinal axis thereof, while envelopedby said coil.

6. In an apparatus as set forth in claim 4, an enclosure having an openside, said base wall when in said position substantially closing saidopen side, a portion of said support and said carrier being received insaid enclosure, and said holder means being adapted to be received insaid closure when said base wall moves away from said position thereof;and air lock means on said enclosure for access thereto.

7. In an apparatus as set forth in claim 4, cooperating abutment meanson said carrier and on said base wall engageable for moving said basewall toward said position thereof when said carrier is moved in saiddirection by said actuating means.

References Cited UNITED STATES PATENTS 2/1961 Emeis. 8/1966 Prediger etal. 13-1 XR

